Perfecting printing can be carried out in a variety of ways. In the case of conventional presses the sheets must be restacked after the first side has been printed in order that the second side can be printed in a second pass. However, this complexity is unsatisfactory when perfecting printing is required frequently. Another possibility is to turn the sheets within the press so that first one side of the paper is printed and, thereafter, the sheet is turned and the second side of the paper can be printed in the next printing unit. However, the cost of such a turning facility is appreciable. Also, a relatively large number of printing units are required depending upon the number of colors required to be printed. Another difficulty of such printing is that after the sheet has been turned, it is conveyed onwards with its back edge at the front. Registration therefore becomes difficult and more particularly a second gripper edge is required for this purpose. Consequently, conveyance of the sheet depends upon a relatively large number of transfers and requires the paper to be cut accurately on all sides.
In the past, efforts have been made for single-step perfecting printing wherein the sheet is printed simultaneously from the top and from the bottom in a printing zone. There have been many variations on this theme. More particularly, the possibility of perfecting printing between two blanket cylinders has been carried into practical effect both in sheet-fed offset printing and in web offset printing. Presses comprising two consecutive complete offset printing units have been disclosed; unfortunately they require a sheet transfer between the first printing and second printing. For the sake of simplification, therefore, the offset impression cylinder has had one or more printing plates placed on it and by alternate changeover has been used as a plate cylinder and also as a backing cylinder for offset printing and as a plate cylinder for direct planography.
In the prior art, DE-PS No. 179 218 discloses a perfecting printing press having two hard printing formes and a backing element and transfer element respectively. The printing unit has a transfer cylinder or backing cylinder and two forme cylinders of the same size. An inking unit and a damping unit for planographic printing are associated with each forme cylinder. The transfer cylinder has a soft surface and is disposed between the two forme cylinders. The paper passes through between the transfer cylinder and the first forme cylinder with the transfer cylinder transferring the indicia of the second forme cylinder to the bottom of the paper--i.e., by indirect transfer as the first forme cylinder acts as backing cylinder for indirect printing. However, the other side of the paper is printed directly by the first forme cylinder with the transfer cylinder being operative from this point of view as a soft backing cylinder. In accordance with the prior printing process, the printing plate on the first forme cylinder must be copied in laterally inverted relationship to the printing plate on the second impression cylinder.
Another prior reference, DE-PS No. 366 371 discloses a rotary lithographic press having a small blanket cylinder and a second cylinder several times larger than the blanket cylinder with both cylinders having grippers. Depending upon the nature of operation and construction, two or three printing plates can be disposed on the second cylinder. Correspondingly, at least one inking device is associated with the second cylinder. If an appropriate arrangement is made, the inked first plate can be copied without paper feed on the blanket of the first cylinder. When the second plate comes into the printing zone, a sheet is supplied. As it passes through the printing zone the sheet is printed in offset by the small cylinder and in direct planography by the large cylinder, the two cylinders working as backing cylinders for the respective other printing process.
It is also recognized as desirable for presses of the kind described that a single cylinder of a press should not serve just one purpose. Correspondingly, printing plates are placed on the pressing cylinders or pressers and the blanket cylinders are used as pressers. It is also known to use a softly covered cylinder as a backing cylinder for direct planographic printing. The problem in these cases is that both the direct planographic printing and the indirect offset printing must proceed under the same conditions--i.e., the press setting selected for pressure and cylinder packings must be satisfactory for both forms of printing.
Printing zone requirements for optimum direct planographic printing are, of course quite different from those for offset printing. Many different factors are concerned here, but the chief point is the fact that in direct planographic printing the paper surface contacts a hard smooth plate surface--i.e., the paper surface must adapt to the surface of the planographic plate--whereas in offset printing the relatively soft blanket surface conforms to the paper surface. The paper surface must therefore be adapted to the plate surface. This can occur with a soft impression cylinder or with a blanket cylinder. This requires a relatively high pressure engagement or pressure setting between the cylinders. If, however, the blanket cylinder is also used to transfer indicia, as occurs in the cases described, there is bound to be interaction between the print conditions of the two processes. On the one hand, if the pressure is adjusted to be suitable for offset printing, the pressure for direct planographic printing will be too low--i.e., the printing may be unsatisfactory due to the paper not being pressed sufficiently tightly on to the planographic plate. Completeness and uniformity of indicia depend upon the smoothness of the paper. On the other hand, if the pressure adjustment is adapted for direct planographic printing--i.e., is higher than the pressure required for offset printing--the pressure for the offset printing is excessive and the ink will be squeezed apart between the blanket and the paper, with the result of an unwanted print widening. In other words, the quality of the print on both sides of the paper is impaired in opposite senses.
A middle ground must therefore be found such that there is very little adverse interaction between the two processes. Also, special unwinding conditions are operative for direct planographic printing since the paper must not be allowed to rub on the printing plate. This may occur if conditions are set up incorrectly in the printing zone where the blanket or backing cylinder makes flexing movements. If the flexing movements result in relative movements of the paper on the printing plate, the plate surface rapidly suffers from damage or even becomes unserviceable.